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Increasingly, upcoming trials for Alzheimer's therapeutics will target prevention—seeing whether drugs can stave off the disease before it takes hold. Such studies need to find cognitively normal people who are on the brink of the disease. Which biomarkers will help zero in on that population? An Archives of Neurology paper published April 23 by Anders Dale, University of California, San Diego, and colleagues confirms that a combination of cerebrospinal fluid (CSF) Aβ42 and phosphorylated tau (p-tau) fits the bill.

"These findings illustrate the need for assessing p-tau, in addition to Aβ, for clinical trial recruitment and design," first author Rahul Desikan told Alzforum in an e-mail.

Desikan and colleagues used data from 107 clinically normal patients enrolled in the Alzheimer's Disease Neuroimaging Initiative (ADNI). Each had CSF samples taken at baseline. They also underwent cognitive analysis then, and three years later. Cognitive tests included the Clinical Dementia Rating Scale (CDR), CDR-Sum of Boxes (CDR-SB), and the Alzheimer's Disease Assessment Scale-Cognitive subscale (ADAS-cog). Based on CSF analysis, the research group categorized subjects as having high (>23 pg/mL) or low (192 pg/mL) or low (Shaw et al., 2009).

By itself, a low CSF Aβ42 level at baseline did predict, on average, cognitive decline in these normal individuals, with all three cognitive test scores dropping. But the scientists picked that relationship apart by examining whether CSF p-tau influenced the interaction. They found that cognitive scores actually fell only in those people who had both low CSF Aβ and high p-tau. If CSF p-tau levels were low, regardless of Aβ level, scores were closer to normal. "If you take p-tau away, there's no effect of Aβ on clinical decline," said Desikan. "This suggests that p-tau is acting like a critical link between Aβ and clinical decline." Similar findings were reported previously (see Fagan et al., 2009).

The findings should help researchers select patients for prevention trials. One of the problems researchers face when conducting those trials is finding at-risk individuals who will progress to dementia before the trial is over. Screening for elevated CSF p-tau levels in addition to lowered Aβ42 may help scientists catch cognitively normal people in the two to five years before they begin to decline, wrote David Holtzman, Washington University, St. Louis, Missouri, in an accompanying editorial. Using such a sample population would allow trials (which normally last only 18 months) a better chance of observing a drug's preventative capabilities.

On the other hand, researchers may prefer to select people who have low CSF Aβ but not yet have elevated tau levels, Desikan said. If tau phosphorylation is initiated by Aβ but then continues independently of Aβ (dependence on Aβ is not yet known), then that would imply that it is too late for Aβ-clearing drugs to stop the neurodegenerative process. The bottom line, he said, is that, for any given study, researchers should know the p-tau status of their participants.

Results were more ambiguous for high total tau (t-tau), which did not link Aβ and cognitive decline, according to CDR and CDR-SB results, though it did in the ADAS-cog. In previous studies, t-tau has been shown to correlate well with p-tau (see Fagan et al., 2011). Differences in CSF assays could cause the discrepancy between this study and previous ones, or p-tau might be a stronger biomarker of AD than t-tau, Desikan said.

Similar to the current findings, Desikan and colleagues last year reported that brain atrophy only occurs in non-demented healthy controls or people with MCI who have both low CSF Aβ42 and high p-tau, not just low CSF Aβ42 (see ARF related news story on Desikan et al., 2011). Other studies hinted at biomarkers other than CSF Aβ that predict cognitive decline in healthy controls such as the p-tau or t-tau to Aβ42 ratio (see Fagan et al., 2007), YKL40 (chitinase-3 like-1; see Craig-Schapiro et al., 2010), or visinin-like protein 1 (see Tarawneh et al., 2011). Taken together, recent evidence suggests that CSF biomarkers other than just Aβ42 will be important for screening in clinical trials.

"There are some in the field who seem to want to select patients based on either CSF amyloid markers or PET scan evidence of amyloid deposition for inclusion in 'prevention' trials," wrote Peter Davies, Albert Einstein College of Medicine in the Bronx, New York, in an e-mail to Alzforum. "This paper and the majority experience of autopsy studies suggest that such a group would have a low likelihood of progression, and that inclusion of p-tau markers would really help."

CSF p-tau, along with Aβ42, probably won't be used in recruitment of secondary prevention trials right away, said Eric Reiman, Banner Alzheimer's Institute, Phoenix, Arizona, because he and other researchers are initially looking for a range of pathology in cognitively normal people, including very early amyloid deposition without neurodegeneration. The presence of p-tau indicates that neurons have already begun to die, he said, and some treatments could be too little too late at that point. But depending on the type of participants needed, later study recruitment will likely use such biomarkers of neurodegeneration, said Reiman.—Gwyneth Dickey Zakaib

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The conclusion that elevated CSF p-tau levels with reduced Aβ levels (Aβ+/p-tau+) are better correlated with clinical decline in baseline CDR = 0 (cognitively normal) individuals will help the pharmaceutical industry/clinical researchers better stratify subjects in secondary prevention trials. For basic researchers, however, this study raises an interesting question. High CSF p-tau levels are considered to be indicative of neurodegeneration. So, what do 19 (out of 107) individuals with elevated p-tau levels but without amyloid pathology (Aβ-/p-tau+) with baseline CDR = 0 represent? The mean age of these individuals is 78 years, making them unlikely to represent frontotemporal dementia with parkinsonism (FTDP) (where mean age of onset is around 49) or other tauopathies. Also, this population remained CDR = 0 at the end of the three-year follow-up period, indicating the neurodegeneration, as indicated by high p-tau does not result in clinical deficits.